Optical counterparts of an ultraluminous X-Ray Source X-1 in NGC 2500

Abstract

ABSTRACT We present the results of a search for optical counterparts of ultraluminous X-ray source (ULX) X-1 in the nearby galaxy NGC 2500 by using archival images taken with the Hubble Space Telescope Wide Field Camera (WFC3)/UVIS. We identified four optical sources as possible counterparts within the 2σ error radius of 0$^{\prime \prime }_{.}$3 in the images. However, only two of them were investigated as candidates for counterparts due to their point-like features and their identification in various filters. These two faint candidates have absolute magnitudes of MV ≈ −3.4 and −3.7. The spectral energy distributions of two candidates were modelled by a power-law spectrum with a photon index (α) ∼1.5, but the spectrum of one candidate shows a deviation. This may suggest that at least two components are responsible for the optical emission. The red part of the spectrum could arise from the companion star and the blue part could be interpreted as an evidence of reprocessing of the X-rays from the disc.